1. Field of the Invention
The present invention relates to a method of plasma etching, a method of forming a plasma and an etching gas composition.
2. Discussion of the Background
A specific application of plasma etching involves etching of a composite substrate in which the stack of a semiconductor device formed on a semiconductor substrate is encased in a first dielectric material such as a silicon nitride having a lower etch rate under conditions which typically etch a bulk dielectric layer such as a silicon oxide which overlies the first dielectric material. Using a technique known as self-aligned contact (SAC) etching, a patterned photoresist is formed, leaving openings overlying a region in the semiconductor device to which an opening is desired. When typical plasma etching conditions are conducted, etching of the bulk dielectric material occurs quite easily, while etching of the first dielectric material is relatively slow, such that even if there is a minor misalignment of the photolithographic mask, formation of an opening to the desired region, without damaging the device stack is possible.
However, high etch selectivity between the first and bulk dielectric materials is more difficult to obtain at smaller contact hole sizes of  less than 0.35 xcexcm, and at a high aspect ratio (i.e.  greater than 4:1) contact hole.
Nulty U.S. Pat. No. 5,468,342 reports a method of etching openings in an oxide layer in which 1,1,1,2-tetrafluoroethane (HFC 134a) is used as an additive to an etchant gas, allowing for improved oxide:nitride selectivity. This mixture offers improved oxide:nitride selectivity relative to CHF3, however such selectivity decreases when forming higher aspect ratio contact openings, and smaller dimension contact holes.
Nulty U.S. Pat. No. 5,562,801, reports a hard mask etching process in which 1,1,1,2-tetrafluoroethane is an additive to the etchant gas to improve oxide:nitride selectivity.
Marks et al U.S. Pat. No. 5,423,945, report an increase in the oxide:nitride etching selectivity by introduction of a scavenger for fluorine, such as a source of silicon ions or graphite. Etchant gases such as CF4, C2F6 and C3F8 are reported.
Collins et al U.S. Pat No. 5,300,460, report a plasma-assisted etching process in which a power source having a frequency range of from 50-800 MHz is used.
Blalock et al U.S. Pat. No. 5,286,344, report a selective etch process in which a fluorinated chemical etch system is used comprising an etchant material and an additive material comprising a fluorocarbon material in which the number of hydrogen atoms is equal to or greater than the number of fluorine atoms.
Tahara et al U.S. Pat. Nos. 5,302,236 and 5,356,515, report an etching process of an object having an oxide or nitride portion by a gas plasma containing a halogen element with a gas containing both carbon in an oxidation state less than 4 and oxygen.
Yanagida U.S. Pat. No. 5,376,234, reports a dry etching method in which the etchant gas comprises a compound selected from mercaptan, thioether and disulfide having a fluorocarbon side chain, as effective for reducing the deposit of polymer.
Present etching methods do not provide entirely satisfactory result when etching small contact holes ( e.g.  less than 4.0 xcexcm) and at a high aspect ratio (e.g. 4:1). Increasing demands on miniaturization as device density for integrated circuits increases are pushing the limits of existing etching technology. Accordingly, improved methods for selectively etching an oxide layer are sought.
One aspect of the present invention is directed to a method of plasma etching a dielectric layer.
Another embodiment of the present invention is directed to a method of forming a plasma.
Another embodiment of the present invention is directed to a plasma etching gas.
These and other aspects of the present invention are made possible by an etching process in which a mixture of first and, optionally second (hydro)fluorocarbon additive gases are added to an etching gas for a plasma etching process. Applicants have discovered that this mixture enhances the selectivity of oxide etching over nitride etching. A (hydro)fluorocarbon gas is a gas which optionally contains hydrogen, and does contain fluorine and carbon.